NHC-Stabilized Au₁₀ Nanoclusters and Their Conversion to Au₂₅ Nanoclusters

Abstract

Herein, we describe the synthesis of a toroidal Au 10 cluster stabilized by N -heterocyclic carbene and halide ligands via reduction of the corresponding NHC–Au–X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au 10 show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au 25 cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au 25 was observed even upon the reduction of NHC–Au–I. The isolated bromide derivative of the Au 25 cluster displays a relatively high ( ca . 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH−π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au 10 cluster. 13 C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.

DFT Insights into the Variety in the Coordination Modes of the Equatorial Halides in [Au13Ag12(PR3)10X8]+ (X=Cl/Br) Clusters

“…Nevertheless, the low energy gap of 9.9 kcal/mol demonstrates the possible isomerization among these [Au 23 Ag 2 (PPh 3 ) 10 Cl 8 ] + configurations. The framework of Cl F1* (the most stable configuration in Figure 6) resembles that of the recently reported single crystal structure of [Au 25 (NHC) 10 Br 8 ] − (NHC denotes N‐heterocyclic carbene ligand) [30] …”

“…[1] The type/number of the exterior ligands play a pivotal role in controlling the atomic-size [2] and physicochemical properties [3] of a variety of metal nanoclusters. For example, the distinct metal-ligand binding strength is proposed to account to the mutual conversion between Ag 35 (SG) 18 and Ag 44 30 (HSG and 4-FTPH are short for glutathione and 4-fluorothiophenol), with the larger-sized Ag 44 cluster favorably formed in presence of the 4-FTP ligands with a weaker binding affinity (compared to that of the SG ligand). [4] In addition, the ligand effect could also significantly affect the chemical reactivity of clusters.…”

From Au11 to Au13: Tailored Synthesis of Superatomic Di-NHC/PPh3-Stabilized Molecular Gold Nanoclusters

“…Furthermore, the electronic properties of the thiolate ligands cannot be very extensively varied, thus hampering the control over AuNC properties, such as the electronic structure of the cluster, its energy levels, and in ultimate analysis its photophysical behavior . Consequently, alternative ligand systems are currently being investigated to stabilize AuNCs, such as phosphines (PR 3 ), − alkynyls, − and N-heterocyclic carbenes (NHCs). − The latter in particular have been recently shown to be very useful for AuNCs stabilization since very strong Au-NHC bonds can be established. ,, Nowadays, a great number of NHC ligands are available, featuring steric and electronic properties that can be extensively and independently varied for this purpose. − …”

“…In 2019, Narouz et al proved indeed that the insertion by ligand substitution of one NHC ligand on the metalloid cluster [Au 11 (PPh 3 ) 8 Cl 2 ]Cl (Scheme A) improves the cluster stability toward thermal decomposition . After this work, other AuNCs stabilized only by NHCs or di-NHCs have been synthesized using instead a direct reduction approach (Scheme B), in which mono- and di-NHC gold­(I) complexes are treated with NaBH 4 to form Au 11 , Au 13 , Au 25 and, more recently, Au 10 clusters. − AuNCs with Au cores exhibiting particularly stable electronic configurations (“superatoms”) are generally produced with the direct reduction approach when NHCs are used as ligands. Among these species, the [Au 13 (NHC) 9 X 3 ] 2+ or [Au 13 (di-NHC) 5 X 2 ] 3+ (X: Br or Cl) icosahedral clusters represent the most common NHC-stabilized AuNCs. ,,, These structures feature peculiar stability imparted by the Au 13 superatomic core, as well as by the chelating properties of di-NHC ligands.…”

Highly Luminescent NHC-Stabilized Au13 Clusters as Efficient Excited-State Electron Donors

“…The photoluminescence quantum yield (PLQY) is a good figure of merit to optimize to make gold nanoclusters good photosensitizers or photocatalysts. NHC-stabilized gold nanoclusters generally reported a relatively high PLQY (>10%). ,,− Crudden’s group was the first to report gold nanoclusters with monodentate NHC ligands with up to 16% PLQY . Zheng’s group unveiled the synthesis of Au 13 NCs protected by bis- N -heterocyclic carbene (bis-NHC) ligands with an observed high PLQY (14.99%) and a long lifetime (2.4 μs) .…”